Technical Field
The embodiments described herein relate to a voided plastics film and also to a formulation for making a voided plastics film and a method of making such a film. The embodiments are particularly, but not exclusively, concerned with composite plastics films or sheets having a voided base or core layer and at least one skin layer, and preferably having an external printable surface, and to synthetic papers produced therefrom.
Description of the Related Art
The term “synthetic paper” is used herein and throughout the specification to mean plastics film and sheet products having a feel and printability similar to cellulose paper. Synthetic paper is a polymeric material, typically based on polyolefins (for example polyethylene or polypropylene), which is stretched and orientated to form a sheet with the stiffness and feel of cellulose paper. It has been recognised that plastics sheet products of these types can provide an improved alternative to cellulose paper where durability and toughness are required. Plastics sheets produced from polyolefins have several advantages over other plastics since they offer UV resistance, good tear strength, water resistance and the ability to be recycled in many post-consumer waste applications.
Synthetic papers have been produced commercially by the plastics industry for many years and have taken a number of different forms. They have included products having voided (i.e. multicellular) or unvoided structures, some of which have been coated with filler- and/or pigment-containing surface coatings to improve printing qualities. The voiding technique has frequently been used to reduce the density of the synthetic paper produced.
As described in EP0703071A a commonly used voiding agent is a zinc-calcium resinate, which causes the voiding when a heated sheet of synthetic paper is stretched. This technique produces a very serviceable sheet which has gained widespread commercial acceptance, particularly when it incorporates white inert fillers. These fillers and the voids typically give rise to sheets with a brightness (ISO 2470) of 80-88%. This range of brightness is due in part to the fact that zinc calcium resinate has a yellow color.
EP0863177A describes an alternative synthetic paper composition in which a pentaerythritol ester of maleic rosin is used as the voiding agent. This composition produces a synthetic paper that is highly satisfactory in terms of its performance and physical characteristics. However, the pentaerythritol ester of maleic rosin used as the voiding agent is very expensive and only a limited supply is available. This limited supply availability is due to the fact that specific characteristics of elevated temperature softening behavior are needed and most readily available versions of pentaerythritol ester of maleic rosin are not suitable as voiding agents. This affects the price of the synthetic paper and the applications for which it is commercially suitable. It can also affect the profit margin of the producer.
It is therefore desirable to make a synthetic paper that matches or exceeds the physical characteristics of existing synthetic papers, but which uses an alternative voiding agent, in particular one that is more readily available and cheaper than a pentaerythritol ester of maleic rosin.